Magnetoresistive material with two metallic magnetic phases

ABSTRACT

A magnetoresistive material with two metallic magnetic phases. The material exhibits the giant magnetoresistance effect (GMR). A first phase of the material includes a matrix of an electrically conductive ferromagnetic transition metal or an alloy thereof. A second precipitate phase exhibits ferromagnetic behavior when precipitated into the matrix and is antiferromagnetically exchange coupled to the first phase. The second precipitate phase can be electrically conductive rare earth pnictide or can be a Heusler alloy. A method of manufacturing magnetoresistive materials according to the present invention employs facing targets magnetron sputtering. The invention also includes a method of detecting magnetic field strength by providing a read head including a portion of one of the magnetoresistive materials according to the invention, exposing the read head to the magnetic field of a magnetic recording medium, sensing electrical resistivity of the portion of material associated with the magnetic field of the magnetic recording medium, and converting the electrical resistivity into a signal which is indicative of the magnetic field strength of the magnetic field associated with the magnetic recording medium. A digital magnetic recording system, according to the present invention, is adapted for use with a magnetic recording medium having a characteristic coercive force and a plurality of stored bits thereon. The bits are stored by magnetic field strength levels of a magnetic field associated with the medium. The system can include a conventional write head and a controller. The system can also include a read head including a portion of magnetoresistive material according to the present invention which is located in proximity to the medium and a suitable resistivity sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to materials for use in magnetic recordingsensors and the like, and more particularly relates to amagnetoresistive material with two metallic magnetic phases.

2. Brief Description of the Prior Art

Materials which exhibit a change in resistance when exposed to amagnetic field are of use in preparing magnetic recording sensors, suchas those used, for example, in computer disk drives. At the presenttime, state-of-the-art computer disk drives employ sensor materialswhich exhibit the anisotropic magnetoresistance effect (AMR). Materialswhich exhibit the AMR have a magnetoresistance which depends on how themagnetic field is applied with respect to the direction of current flow.

In other types of materials, which exhibit the giant magnetoresistanceeffect (GMR), a non-magnetic metallic conductor, such as copper, isnecessary to create a disordered state of electron spins in aferromagnet. However, the nonmagnetic metallic conductor does notcontribute to desired scattering of the conduction electrons, and infact, may act as a low resistance shunt path which decreases themagnetoresistance. These prior GMR materials include a magnetic metaland a non-magnetic metal.

Macroscopic ferrimagnets are a new class of phase separated magneticmaterials which have been recently discovered, and are described, forexample, in R. J. Gambino et al., 75 J. Appl. Phys. 1871 (1994). Themacroscopic ferrimagnets include two magnetic phases with a negativemagnetic exchange at the phase boundary. A prototypical example is theCo—EuS system which has 100 Å particles of EuS in a cobalt matrix. TheEuS is exchange coupled antiferromagnetically to the cobalt at least atthe Co/EuS interface. In the Co—EuS system, the small size of the EuSparticles results in a large fraction of the EuS being in closeproximity to the interface which is influenced by the strong Co/EuSexchange. It has been found that these materials display unusualmagneto-optical properties, as described in R. J. Gambino and P.Fumagalli, 30 IEEE Trans. Magn. 4461 (1994), and magneto-transportproperties, as described in R. J. Gambino and J. Wang, 33 Scr. Metall.Mater. 1877 (1995) and R. J. Gambino et al., 31 IEEE Trans. Magn. 3915(1995). Magnetization and Kerr hysteresis loops have confirmed themacroscopic ferrimagnetic model for these systems. In measurements ofthe optical and magneto-optical properties of Co—EuS thin films, polarKerr rotations of up to 2° have been observed in Co-rich films at photonenergies of 4.5 eV, as described in P. Fumagalli et al, 31 IEEE Trans.Magn. 3319 (1995). Transport measurements show that themagnetoresistance of Co—EuS behaves like that of the widely studiedgranular giant magnetoresistance effect (GMR) materials, as described inS. Zhang, 61 Appl. Phys. Lett. 1855 (1992), which include particles of aferromagnetic metal in a conductive, nonmagnetic matrix. In contrast,Co—EuS includes semiconducting, ferromagnetic particles in a conductive,ferromagnetic matrix of cobalt. As a consequence, the temperaturedependence of the magnetoresistance is very different in the Co—EuSsystem as compared to the ordinary granular GMR materials. With respectto the magnitude of the effect, the magnetoresistivity change (δρ) ofthe Co—EuS system is 8×10⁻⁵ Ω-cm at room temperature in a field of 1T,which is larger than other magnetoresistive materials. Even though themagnetoresistivity change of this system is large, the magnetoresistancedefined as δρ/ρ is small, typically 2˜3%, because of the highresistivity of the material caused by a large volume fraction of thesemiconducting EuS phase.

While materials exhibiting the AMR effect have enhanced the performanceof computer disk drives, and while the aforementioned Co—EuS systems arepromising, it would be desirable to develop materials having a largerchange in resistance as a function of applied magnetic field strength,that is, a larger magnetoresistance effect. Such materials could permitthe development of more sensitive magnetic recording sensors. It wouldbe desirable to develop such materials which would exhibit the GMR asopposed to the AMR. In materials exhibiting the GMR, the resistivitydecreases with the applied magnetic field independent of the directionof the applied field with respect to the direction of current flow. Inaddition to this desirable isotropy, the GMR is usually stronger thanthe AMR.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved materialsuitable for manufacturing more sensitive magnetic recording sensors.

It is another object of the present invention to provide such a materialwhich exhibits the GMR.

It is a further object of the present invention to provide such amaterial which includes two ferromagnetic phases which are exchangecoupled antiferromagnetically.

It is yet another object of the present invention to provide a method ofmanufacturing such a material.

It is a further object of the present invention to provide a method ofsensing magnetic fields using such a material.

It is still another object of the present invention to provide a digitalmagnetic recording system which utilizes such a material in a read head.

In accordance with one form of the material of the present invention, amagnetoresistive material exhibits the GMR and has two phases. The firstphase includes a matrix of an electrically conductive ferromagnetictransition metal or an alloy thereof. The second phase is a precipitatephase of an electrically conductive rare earth pnictide which exhibitsferromagnetic behavior when precipitated out of the matrix. The secondphase is antiferromagnetically exchange coupled to the first phase. In apreferred form of the first embodiment, the matrix comprises cobalt andthe precipitate phase comprises terbium nitride.

Thus, the present invention provides a new macroscopic ferrimagnet, inthe system Co—TbN, which exhibits the GMR. The Co—TbN systemdemonstrates typical macroscopic ferrimagnet properties: a magneticcompensation point and negative GMR. The Co—TbN system with 32% TbN byvolume composition shows 0.72% GMR under an applied field of 8 kOe atroom temperature and 9% GMR at 250° K under an applied field of 40 kOe.In the Co—TbN system, the temperature dependence of the GMR is quitedifferent from that of ordinary GMR materials, where the negativemagnetoresistance decreases with increasing temperature. The GMR in theCo—TbN system increases with increasing temperature, which is due to theincrease of ferromagnetic alignment of the Co and TbN with an appliedfield caused by the decrease of exchange coupling by temperature.

In an alternative form of material according to the present invention,the second precipitate phase comprises an electrically conductiveHeusler alloy such as Co₂MnSn or Co₂TiSn.

The present invention also provides a method of manufacturing amagnetoresistive material of the types described above. The methodincludes the steps of providing a target (for example, a sheet metal orthin film target) of an electrically conductive ferromagnetic transitionmetal or an alloy thereof; locating a plurality of pellets of anelectrically conductive rare earth element (or constituents of a Heusleralloy) on a surface of the target; sputtering the target and the pelletswith ions in a suitable plasma, such as an argon plasma, to cause thefilm and the pellets to form an amorphous alloy of the electricallyconductive ferromagnetic transition metal or alloy thereof, and theelectrically conductive rare earth element (or constituents of a Heusleralloy); and subsequently annealing the amorphous alloy to causeformation of the precipitate phase within the matrix of theferromagnetic transition metal or alloy thereof. Techniques other thansputtering can also be employed.

The present invention further provides a method of detecting magneticfield strength of a magnetic field associated with a magnetizationpattern recorded in a medium. The method includes the steps of providinga sensing head which includes a portion of a magnetoresistive materialof the type described above; exposing the sensing head to the magneticfield of the magnetization pattern in the magnetic recording medium;sensing the electrical resistivity of the portion of magnetoresistivematerial exposed to the magnetic field of the magnetization pattern inthe medium; and converting the electrical resistivity of the portioninto a signal which is indicative of the magnetic field strength of themagnetization pattern in the medium. It will be appreciated that any ofthe materials of the present invention described above can be used forthe portion of magnetoresistive material in the read head.

The present invention yet further provides a magnetic recording systemwhich is adapted for use with a magnetic recording medium having acharacteristic coercive force and which has a plurality of stored datain it. The data is stored in the form of a magnetization pattern in themedium. The magnetic recording system includes a write head (which isoptional) and a read head. The read head includes a portion of themagnetoresistive material of the present invention which is located inproximity to the magnetic recording medium and also includes aresistivity sensor which detects the resistivity of the portion ofmaterial according to the present invention corresponding to magneticfield strength levels of the magnetization pattern in the medium. Thesystem also includes a controller which controls the read head (andoptional write head) and which converts the detected resistivity of theportion of material according to the present invention into a signalwhich is indicative of the stored data in the recording medium. Again,the portion of material can be any of the magnetoresistive materialsaccording to the present invention.

These and other features and advantages of the present invention will bepointed out in the following specification, taken in connection with theaccompanying drawings, and the scope of the invention will be set forthin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of percent change in resistivity (GMR) against appliedmagnetic field at room temperature for a Co_(0.68)—(TbN)_(0.32) materialaccording to the present invention;

FIG. 2 is a graph of resistance versus temperature at two differentvalues of applied magnetic field strength for the material of FIG. 1;

FIG. 3 is a plot of percent change in resistivity (GMR) vs. the volumepercent of terbium nitride present in a sample;

FIG. 4 is a plot of percent change in resistivity (GMR) vs. appliedmagnetic field at room temperature for a system employing a Co/Co₂TiSnthin film annealed in a vacuum;

FIG. 5 is a figure similar to FIG. 4 except showing results for thematerial when annealed in nitrogen;

FIG. 6 is a schematic diagram of a facing targets magnetron sputteringsystem suitable for producing materials in accordance with the presentinvention;

FIG. 7A is a diagram showing a uniform magnetic field in the system ofFIG. 6;

FIG. 7B is a diagram showing a divergent magnetic field in the system ofFIG. 6;

FIG. 8 is a diagram of deposition rate against argon pressure atdifferent substrate distances;

FIG. 9 is a plot of composition vs. argon pressure at differentsubstrate distances;

FIG. 10 is a plot of sputter yield ratio of terbium to cobalt againstargon pressure;

FIG. 11 shows an x-ray diffraction pattern of a TbCo film annealed at650° C. in a nitrogen gas atmosphere;

FIG. 12 shows a digital magnetic recording system which employsmaterials of the present invention;

FIG. 13 shows the magnetization of Co_(0.68)—(TbN)_(0.32) as a functionof temperature at various applied magnetic fields;

FIG. 14 shows the magnetization of a Co—TbN macroscopic ferrimagnet withapplied magnetic field at various temperatures;

FIG. 15A shows the change in resistivity of a Co_(0.68)—(TbN)_(0.32)film as a function of magnetic field at room temperature;

FIG. 15B shows the changes in resistivity for the same film, also as afunction of applied magnetic field, at room temperature; and

FIG. 16 is a plot of the percent change in resistivity (GMR) of aCo_(0.68)—(TbN)_(0.32) film as a function of temperature in a relativelyhigh magnetic field of 40 kOe.

DETAILED DESCRIPTION OF THE INVENTION

One form of material in accordance with the present invention is amagnetoresistive material exhibiting the giant magnetoresistance effect(GMR) and which has two phases. The first phase comprises a matrix of anelectrically conductive ferromagnetic transition metal or an alloythereof. The second phase is a precipitate phase of an electricallyconductive rare earth pnictide which exhibits ferromagnetic behaviorwhen precipitated out of the matrix. The second phase isantiferromagnetically exchange coupled to the first phase.

The electrically conductive ferromagnetic transition metal or alloythereof can include at least one of iron, cobalt and nickel or an alloyof one or more of those elements, such as FeCo, FeNi, NiCo, CoNiFe,NiCu, FeCr, CoAl, and the like. The alloys can be formed from theferromagnetic metal and non-magnetic materials (e.g., the NiCu, FeCr,and CoAl). Ferromagnetic manganese and chromium compounds can also beemployed. The rare earth pnictide can include a rare earth element (or acompound or alloy) thereof, selected to anti-parallel couple to thetransition metal or alloy thereof, and (in turn) compounded with one ofnitrogen, phosphorous, arsenic, antimony and bismuth. Terbium ispresently believed to be the preferred rare earth element.

The precipitate phase may exhibit independent ferromagnetic behavior,that is, it may behave ferromagnetically by itself. Alternatively, theprecipitate phase may not show any meaningful independent ferromagneticbehavior, but may become ferromagnetic, that is, may exhibit meaningfulferromagnetic behavior, when it is precipitated into the matrix. In thislatter case, the exchange coupling between the phases will boost theferromagnetism in the small particles of the precipitate phase.

In a preferred material according to the present invention, the matrixcomprises cobalt, and the precipitate phase comprises terbium nitride.The terbium nitride should be present in a volume percent which issufficient to produce the GMR in the material without causingundesirable discontinuities in the cobalt matrix. That is to say, thecobalt should be substantially continuous throughout the device: thereshould not be cobalt regions surrounded by TbN. The cobalt matrix canhave a volume percent of from about, for example, 30% to about 75%. Theterbium nitride precipitate phase can have a volume percent of fromabout 25% to about 70%. As presently understood, it is believed that atabout 70% terbium nitride, the cobalt matrix would begin to have theaforementioned undesirable discontinuities.

Preferably, the cobalt matrix has a volume percent of from about 61% toabout 70%, and the terbium nitride precipitate phase has a volumepercent of from about 30% to about 39%. As discussed below, tests wereconducted from about 31 volume percent to about 39 volume percent ofterbium nitride and indicated that the GMR continued to increase withincreasing volume percentage of terbium nitride. However, the rate ofincrease appeared to lower between about 35% and about 39% volumepercent of terbium nitride.

The Co—TbN system is a macroscopic ferrimagnet and has TbN precipitatesin a Co matrix. The TbN has the same magnetic moment as pure Tb and therock salt structure, as described in R. J. Gambino and J. J. Cuomo, 113J. Electrochem. Soc. 401 (1966), the same as EuS. The TbN precipitatesalso provide the higher Curie temperature and thus stronger antiparallelexchange coupling with the Co matrix than EuS. These stronger exchangeeffects are caused by conduction electron mediated exchange of the RKKYtype, as described in C. Kittlel, Introduction to Solid State Physics628 (7^(th) Ed., John Wiley and Sons, 1996), which is weak insemiconducting EuS. Another difference is the single ion anisotropy ofthe Tb ion which is a non-S-state ion. In contrast, EuS containsdivalent europium which is a S-state ion and thus has zero single ionanisotropy. Furthermore, the TbN is a conductor rather thansemiconductor so the resistivity of Co—TbN is much less than that ofCo—EuS, which can improve the magnetoresistance, δρ/ρ. The Co—TbNdiffers with the granular GMR materials in that both phases are magneticand also differs from the Co—EuS in that both phases are conductors.

In an alternative type of material according to the present invention,the first phase is substantially similar to the first phase discussedabove, and the second precipitate phase can include an electricallyconductive Heusler alloy which exhibits ferromagnetic behavior whenprecipitated out of the matrix. Again, the second phase isantiferromagnetically exchange coupled to the first phase. In bothcases, a preferred material for the matrix is cobalt. The precipitatephase can be, for example, Co₂MnSn or Co₂TiSn. Compositions in the rangeof 16.7 to 50 mole percent Co₂MnSn in a Co matrix and 16.7 to 50 molepercent Co₂TiSn in a Co matrix have been tested. In both systems, the 50mole percent compositions are currently believed to be preferable. Themole % is determined by dividing the number of moles of Co₂MnSn orCo₂TiSn by the total number of moles and multiplying by 100. Forexample, Co₃MnSn=Co/Co₂MnSn=50 mole % Co₂MnSn and Co₇MnSn=5Co/Co₂MnSn=16.7 mole % Co₂MnSn. Other operable ranges of thecomponents, exhibiting desirable GMR properties, which can be easilyascertained by one skilled in the art, are within the scope of theinvention. The aforementioned discontinuities in the cobalt matrixshould be avoided in the Heusler alloy systems as well.

Reference should now be had to FIG. 1 which plots the GMR againstapplied magnetic field strength for a film made of a material accordingto the present invention which includes 68 volume percent cobalt and 32volume percent terbium nitride. The horizontal axis is the appliedmagnetic field strength H, in oersteds, while the vertical axis showsthe GMR as measured by the change in resistivity divided by theresistivity in the saturated state, ρ(H_(sat)), expressed as a percent.More specifically:

GMR=δρ/ρ=[ρ(0)−ρ(H _(sat))]/ρ(H _(sat)),  (1)

where ρ(0) and ρ(H_(sat)) are the resistivities in zero field and in asaturating magnetic field, respectively.

FIG. 2 shows a plot of the resistance of a sample of the same materialas FIG. 1, measured in ohms, as a function of temperature measured indegrees Kelvin with an applied field of H=40 kOe and with an appliedfield of H=0. In determining the GMR, using the equation R=ρL/A whereR=resistance, ρ=resistivity, L=length, and A=cross sectional area,ΔR/R=Δρ/ρ for a given geometry.

FIG. 3 is a plot of the GMR, again expressed as a percent change inresistivity, against the volume percent of terbium nitride in thecobalt-terbium nitride material. As can be seen, the GMR continues toincrease with increasing volume percentage of terbium nitride but therate of change of the increase is less at the higher volume percentagesof terbium nitride, that is, about 35-39 volume percent of terbiumnitride.

FIG. 4 shows a plot of GMR, again as a percent change in resistivity,versus the applied fields in oersteds for a Co/Co₂TiSn thin filmannealed in vacuum. The plot is in the form of a series of points. FIG.5 is a similar plot but wherein the thin film has been annealed innitrogen gas. It can be seen that the GMR increases significantly whenthe annealing is performed in the nitrogen gas. Similar results areexpected for the materials employing Heusler alloys.

The materials described above are macroscopic ferrimagnets whichcomprise two ferromagnetic phases which are antiferromagneticallyexchange coupled. Both of the ferromagnetic phases are metallicconductors. The electrical resistivity of the materials decreases when amagnetic field is applied, and the change in electrical resistance isindependent of the direction of the magnetic field with respect to thedirection of the current flow. As noted, this type of behavior ischaracteristic of the giant magnetoresistance effect (GMR) which haspreviously only been observed in metallic materials comprising aferromagnetic metal such as cobalt and a nonferromagnetic metal such ascopper. The giant magnetoresistance depends on scattering of conductionelectrons by spins in a ferromagnet in a disordered state. The prior artmaterials employed a nonmagnetic metallic conductor such as copper whichwas necessary to create the disordered state but which did notcontribute to the scattering. As noted, depending on the geometry, thecopper conductors might in fact act as low resistance shunt paths whichdecrease the magnetoresistance. In the materials of the presentinvention, spin scattering can occur in both ferromagnetic phases sothat larger GMR effects are possible, in turn enabling more sensitivemagnetic recording sensors.

The GMR effect for an exemplary composition is about 8% at roomtemperature in a field of 4 teslas and it increases slightly withdecreasing temperature down to 20 degrees kelvin. FIG. 2 shows data fromabout 40° K to about 240° K for H=0 and H=40 kOe. It will be appreciatedthat, in c.g.s. units, B=H+4πM_(s), where B=magnetic flux density inkilogauss, H=magnetic field strength in kilooersteds and M_(s)=magnetization in kilogaus. Outside a ferromagnet, B in kilogauss isnormally equal to H in kilooersteds and thus 40 kOe implies 40kilogauss=4T.

Methods of manufacturing the materials of the present invention will nowbe described. The magnitude of the GMR and the field sensitivity of thematerials can be controlled by post-deposition heat treatment as setforth below in the Example. Reference is now made to FIG. 6, whichdepicts a representative sputtering apparatus, to be used in accordancewith the present invention, designated generally as 8. A method ofmanufacturing a magnetoresistive material exhibiting the giantmagnetoresistance effect (GMR) and having two phases, in accordance withthe present invention, includes the step of providing a suitable target(e.g., sheet metal, a disk, sheet, or plate) of an electricallyconductive ferromagnetic transition metal or an alloy thereof. Thetarget is designated as item 10 in FIG. 6. The method includes theadditional step of locating a plurality of pellets 12 of an electricallyconductive rare earth element on a surface 14 of the target 10. In apreferred form of the present invention, a second target 16 (which canalso be, e.g., a disk, sheet, or plate), also of an electricallyconductive ferromagnetic transition metal or alloy thereof, is employed.The process illustrated in FIG. 6 is known as facing target magnetronsputtering (FTMS) with a composite target. The second target caninitially be a pure conductive ferromagnetic transition metal or alloythereof without any pellets and unalloyed with the rare earth element.FTMS with pellets on both targets (i.e., both targets are compositetargets) is known in the art, as described in Naoe et al., 23 IEEE,Trans. Magn. 3429 (1987).

The method also includes sputtering the target 10 (and target 16, whenpresent) and the pellets 12 so as to cause the targets 10 and 16 and thepellets 14 to form an amorphous alloy on substrate 18. The sputteringcan be performed using a suitable plasma of a noble gas such as argon(preferred), helium or krypton in a plasma region 20.

Other physical vapor deposition (PVD) techniques can be employed besidessputtering, for example, co-evaporation or ion beam deposition.Sputtering is believed to be preferable. Further, chemical vapordeposition (CVD) can also be performed. Using gases of organometallicprecursors of Co and Tb and decomposing them to deposit the metals.

The method can also include the step of annealing the amorphous alloy inan atmosphere containing nitrogen to form a nitride. Other gases such asphosphine, PH₃, or arsine, AsH₃, are used to form phosphides orarsenides, respectively. Gaseous vapors of the elements P, As, Sb or Bican also be used. It is desirable to exclude oxygen and have a slightlyreducing atmosphere. The nitrogen family element is used to bond withthe rare earth element in the amorphous alloy and thus form aprecipitate phase of a rare earth pnictide within a matrix of theferromagnetic transition metal or alloy thereof. One suitable atmosphere(for nitride formation) in which the annealing can be carried out isso-called forming gas which is a mixture of 90% nitrogen and 10%hydrogen by volume. It should be appreciated that any suitable gascontaining nitrogen or any other element selected from column VA of theChemical Abstracts Service (CAS) version of the periodic table, or acompound thereof, can be employed. The annealing time required varieswith the temperature and in general will be less at higher temperatures.For example, for nitrogen, 10-12 hours at 650° C. is sufficient. At 450°C., a detectable reaction occurs in 12 hours but is not complete.

Still with reference to FIG. 6, the illustrated apparatus 8 can alsoinclude first and second magnets 22, 24. The density of the plasma inthe plasma region 20 is enhanced by the presence of the magnetic fieldinduced by magnets 22, 24. The dotted line 26 represents a common axisfor the magnets 22, 24.

Reference should now be had to FIGS. 7A and 7B which depict close-upviews of the magnets 22, 24 with certain other elements of the apparatus8 omitted for clarity. In FIG. 7A, a configuration is shown whereinopposite magnetic poles of magnets 22, 24 face each other. For example,a north pole of magnet 22 is shown facing a south pole of magnet 24. Themagnetic field produced by magnets 22, 24 in the configuration shown inFIG. 7A is represented by lines of force 28. In the case shown in FIG.7A, the plasma density in the plasma region 20 is enhanced by themagnetic field produced by magnets 22, 24. The field is axial from topto bottom, generally along axis 26. The field helps to confine theelectrons produced during the sputtering process to the plasma region20. These electrons collide with the atoms of the gas which forms theplasma, for example argon, to form additional ions, for example argonions. This process is known as electron collision ionization.

As is known in the sputtering art, the ions hit the targets 10, 16 andcause the emission of atoms as well as secondary electrons. In order tomaximize yield, it is undesirable that the secondary electrons hit asurface at a large negative potential. Accordingly, it is desirable toexpose the secondary electrons to both a confining magnetic field asshown in FIG. 7A and to a suitable electric field. As is well known,electrons exposed to electric and magnetic fields move in spiral pathsbetween collisions. Referring back to FIG. 6, a suitable electric fieldcan be applied from a radio frequency (RF) supply 30 connected to targetholders 32, 34 through a suitable matching network 36. RF supply 30 canhave any appropriate power level and frequency known to those skilled inthe plasma sputtering art. For example, a power of approximately 2 kWcan be employed. In the United States, a frequency of 13.56 MHZ, asallocated by the government for scientific experimentation, can beemployed.

Referring back now to FIG. 7A, the magnetic field produced by magnets22, 24 also serves to confine the plasma to a more limited plasma region20 which results in more frequent collisions between electrons andneutral gas atoms, a more intense plasma, and generation of more ionsfor bombardment of the targets. This in turn leads to a higherdeposition rate for the material on substrate 18.

Note that the apparatus 8 can include first and second ground shields38, 40, as known in the sputtering art for purposes of preventingbombardment of the fixtures. Further, a suitable load lock chamber 42can be employed. Rare earth elements oxidize easily. Although theoxidation could be removed by sputtering, the removed oxide would thentend to deposit on the substrate 18, which would be undesirable. Loadlock 42 can be employed to minimize or eliminate oxygen contamination.The substrate 18 is placed into the load lock, the load lock is thenevacuated and then the load lock is connected to the main chamber whichcontains the plasma region 20.

Under certain conditions, it may be desirable to bombard the substrate18 where the desired film is formed. In conventional processes, a biasvoltage can be applied to the substrate. This need not be done with thepresent invention. A self bias, due to the plasma potential, acceleratespositive ions towards all surfaces at ground potential.

In the present invention, bombardment of substrate 18 can be controlledby adjusting the distance L_(s) between the magnet center line axis 26and the substrate 18. Different properties are produced in the finishedproduct depending on whether or not the substrate 18 is itselfbombarded. The composition, magnetic anisotropy, density and electricalresistivity can be controlled by ion bombardment of the substrate duringdeposition of the material. Refer to the discussion of FIGS. 8 and 9below.

As was stated above, target 10 is employed together with pellets 12,while second target 16, when employed, is normally formed only of theelectrically conductive ferromagnetic transition metal. However, in thefacing target magnetron sputtering process depicted in FIG. 6, sincetargets 10 and 16 face each other, there will be a target-to-targetinterchange. Accordingly, the rare earth element, even though initiallyonly on the bottom target 10, will eventually achieve some steady-statelevel in both targets due to inter-target exchange.

Throughout this discussion, it should be noted that the electricallyconductive ferromagnetic transition metal or alloy thereof can be any ofthe metals or alloys discussed above, including, for example, cobalt.Further, the rare earth element can be, for example, terbium, or anyother suitable rare earth element.

Referring again now to FIGS. 7A and 7B, the magnet configuration shownin FIG. 7A is conventional in facing target magnetron sputtering. It isbelieved that this configuration is preferred for production since itwill result in a relatively high deposition rate. FIG. 7B shows analternative configuration for the magnets 22, 24. In this case, likepoles (illustrated as both north poles, but alternatively could be bothsouth poles) face each other. This configuration has the undesirableeffect of allowing secondary electrons to escape from the plasma region,thus lowering the deposition rate. However, it can be used to induceperpendicular anisotropy. This is because the growing film on thesubstrate 18 would be bombarded due to the divergence in the secondaryelectron paths caused by the alternative lines of magnetic force 44 asshown in FIG. 7B. Thus, it will be appreciated that the secondaryelectrons in FIG. 7A describe a confined path, while those in FIG. 7Bare not confined and would tend to follow a path as designated by arrow46

The sputtering process just described can be performed so as to yieldthe amorphous alloy having a composition of from about 10 atomic %terbium to about 50 atomic % terbium, with the balance comprisingcobalt, for example. The parameters to produce desired percentages ofthe rare earth element and the ferromagnetic transition metal will bediscussed further with respect to FIG. 10 below.

The aforementioned method for manufacturing the materials containing therare earth element can also be adapted to manufacture the materialscontaining the Heusler alloys. For example, Co, Mn and Sn or Co, Ti andSn can be co-sputtered and then annealed to precipitate Co₂MnSn orCo₂TiSn respectively. The annealing or heat treating step can beperformed, for example, in vacuum or an inert atmosphere (e.g., flowingargon) and the like. The forming gas annealing atmospheres mentioned forthe rare earth containing materials can be employed. Throughout thisapplication, for both the rare-earth-containing embodiments and theHeusler alloy embodiments, it is to be understood that “precipitatedout” generally refers to the precipitation, out of an alloy, of thesecond phase referred to, as described throughout the application.

Other methods of forming precipitates are also within the scope of theapplication. For example, rapid liquid quenching could be employed toform an amorphous ribbon, wherein a molten mixture is cooled at a veryhigh rate, for example, about 10⁶ C/sec for example, by impinging on awater cooled copper surface or wheel. Subsequent annealing is thenrequired to from the precipitate. Further, the aforementioned vapordeposition techniques can include decomposition of carbonyl compoundssuch as Fe(CO)₅ to metals. A suitable mixture of metal carbonyl can bedecomposed to form an amorphous alloy which precipitates a magneticphase out of a magnetic metallic matrix by annealing.

In the present invention using facing target magnetron sputtering(FTMS), the magnetic field can be divergently applied to the plasma inplasma region 20, which induces a high plasma potential as secondaryelectrons escape to the chamber wall and other grounded parts of thesystem (refer to FIG. 7B). When amorphous TbCo thin films are depositedin a region of high plasma exposure in FTMS, the deposition ratesexhibit unusual changes with Ar pressure. The changes of deposition ratewith pressure in the films deposited at different substrate distancesfrom the system center line (L_(s)) are shown in FIG. 8. Circlesrepresent L_(s)=5.4 cm and squares represent L_(s)=6.4 cm. Thedeposition rate (Angstroms per minute) of the films deposited in the lowplasma-exposed region linearly increases with Ar pressure. In the filmsdeposited in a high plasma-exposed region, the changes of depositionrates are not significant in the Ar pressure range from 6 mTorr to 12mTorr. The difference between the pressure trend line and the observedrate is called Δr_(dep).

FIG. 9 shows the film composition changes with Ar pressure at differentsubstrate distances from the system center line (L_(s)). The Tb contentof the films deposited in a low plasma-exposed region increases with Arpressure. On the other hand, the Tb content in the films deposited in ahigh plasma region suddenly decreases and increases in the Ar pressurerange which shows unusual changes of deposition rate. On the basis ofquantitative analysis, the unusual changes are due to the preferentialresputtering of Tb atoms in amorphous Tb_(x)Co_(1−x) thin films. Adiscussion of resputtering can be found in R. J. Gambino & J. J. Cuomo,15 J. Vac. Sci. Technol. 296 (1978). Resputtering refers to the emissionof atoms from the material forming on the substrate 18 due to ionbombardment thereof The sputter yield ratio of Tb atoms to Co atoms inthe resputtered films were estimated using the model proposed by Harperand Gambino, as described in J. M. E. Harper & R. J. Gambino, CombinedIon Beam Deposition and Etching for Thin Film Studies, 16 J. Vac. Sci.Technol. 1901 (1979).

The Harper and Gambino model is known to those of skill in the plasmasputtering art. It provides a model for determining the composition ofthe accumulating film A_(y)B_(1−y) of two components on a target whichis undergoing resputtering. The value of y is determined from theequation:

y=[α+(α²+4xβ)^(½)]2β,  (2)

where:

α is a parameter given by:

α=(z+xε _(r) −x−ε _(r) z−1),  (3)

β is a parameter given by:

β=(z+ε _(r)−ε_(r) z−1),  (4)

x is the target composition A_(x)B_(1−X) and z is the fractionresputtered given by:

z=(R _(A) +R _(B))/(F _(A) +F _(B))  (5)

where:

R _(A)=ε_(A) JY,  (6)

R _(B)=ε_(B) J(1−Y),  (7)

where ε_(A) is the sputter yield of component A in the film; ε_(B) isthe sputter yield of component B in the film; J is the flux of theetching beam which bombards the substrate on which the film grows(ions/cm²/sec); F_(A) is the atom flux of component A arriving at thesubstrate (atoms/cm²/sec); F_(B) is the atom flux of component Barriving at the substrate; and ε_(r) is the ratio of sputter yieldsdefined by:

ε_(r)=ε_(A)/ε_(B).  (8)

The fraction resputtered is strongly dependent on Ar pressure in FTMSwith a divergent magnetic field, as seen in FIG. 7B. In order to applythe above-discussed Harper and Gambino model to resputtering effects inFTMS with a composite target, certain modifications must be made. Morespecifically: the parameter x in equations (2)-(4) must be redefined asfilm composition without resputtering instead of target composition; andthe parameter ε_(r) used in equations (3) and (4) above and defined inequation (8) must be redefined as the ratio of sputter yields in theresputtered film instead of the ratio of sputter yields. Thus, ε_(r) inthe model for the present invention is defined by:

ε_(r)ε_(r) ^(A)/ε_(r) ^(B),  (9)

where ε_(r) ^(A) and ε_(r) ^(B) are the sputter yields in theresputtered film of components A and B respectively.

Therefore, combining the above equations, the sputter yield ratio,ε_(r), can be expressed as a function of compositions of the films withand without resputtering (x and y) and fraction resputtered (z):

ε_(r)=ε_(r) ^(Tb)/ε_(r) ^(Co)=[(y+x)(1+x−z)+y ²(z−1)]/[(y+x)(x−z)+y²(z−1)],  (10)

where ε_(r) ^(Tb) and ε_(r) ^(Co) are sputter yields of Tb and Co,respectively. The fraction resputtered is obtained from the graphshowing the unusual changes of deposition rate with pressure at FIG. 8.The fraction resputtered (z) can be defined as the ratio of thedeviation of the deposition rate from an ideal incremental line(Δr_(dep)) to the ideal deposition rate at a certain pressure(r_(ideal)), at which it is assumed that the ideal deposition rate lineis parallel to the other deposition rate line without resputtering. Thefilm composition with and without resputtering can be determined fromthe graphs on Tb contents with Ar pressure at different depositiondistances. The values of parameters used to obtain resputter yield ratiousing Eq. (9) are shown in Table I below.

TABLE I Pressure (mTorr) 5.6 7.3 8.3 9.6 11.6 12.6 Film composition (x)0.256 0.261 0.263 0.27 0.29 0.293 Film composition (y) 0.246 0.22 0.2260.257 0.281 0.288 Fraction resputtered (z) 0.09 0.125 0.13 0.13 0.1250.1

FIG. 10 exhibits sputter yield ratio of Tb to Co with pressures at whichresputtering takes place. The resputter yield ratio of Tb to Co are muchhigher than 20 in the range of pressure at which resputtering takesplace. Therefore, it can be seen that the resputtering effect inamorphous Tb_(x)Co_(1−x) thin films is mainly caused by Tb atoms. Theresputter yield ratio has the largest value at 9.6 mTorr Ar pressurewith high plasma potential and high plasma density. Note that,throughout the foregoing, if a ferromagnetic transition metal other thanCo or a rare earth element other than Tb were employed, a similarprocedure could be used for all calculations, using differentsuperscripts on the variables ε_(r) ^(Tb) and ε_(r) ^(Co), for example.

FIG. 11 shows the x-ray diffraction pattern of TbCo film annealed at650° C. in a nitrogen gas atmosphere. Sharp diffraction peaks areobserved showing that the material is crystalline, not amorphous. Thepeaks correspond to diffraction from the (200), (220) and (311) crystalplanes of TbN. Several peaks of face centered cubic (fcc) and hexagonalclose packed (hcp) cobalt are also observed showing that crystallinecobalt is present. The diffraction peaks of (hcp) cobalt are sharpindicating larger crystallites.

Reference should now be had to FIG. 12 in connection with a magneticrecording system of the present invention and a method of detectingmagnetic field strength according to the present invention. The magneticfield strength can, in some embodiments, correspond to digitalinformation stored in a magnetic recording medium. Analog systems arealso possible. As shown in FIG. 12, a magnetic recording system,designated generally as 100, is adapted for use with a magneticrecording medium 102 having a coercive force and adapted to store data(for example, digital data in the form of a plurality of bits (notamenable to illustration)). The bits or other digital or analog data arestored as a magnetization pattern in the medium 102; for example, themagnetization pattern may be stored so as to give rise to a z componentof the H field at two different field strengths to represent binarylogic levels. Medium 102 is shown as a tape-like element forconvenience, but it should be understood that medium 102 could also be adisk element of a disk drive, or the like.

System 100 can include a write head 104 for writing the bits to themedium 102 by producing a magnetic field of N→S or S→N polarity whichexceeds the coercive force of the medium 102. Write head 104 can be of aconventional type made of a magnetic material and having a plurality ofwindings 106 and an air gap 108. In some embodiments of the presentinvention, the write head 104 can be eliminated when it is desired touse the present invention in a read-only situation, for example, in aplayback-only type of video cassette recorder.

System 100 further includes a read head designated generally as 110.Read head 110 includes a portion 112 of a material exhibiting the GMR ofany of the types discussed in this application. The portion 112 ofmagnetoresistive material is located in proximity to the medium 102.Portion 112 is coupled to a resistivity (or resistance) sensor 114 whichdetects changes in resistivity (or bulk resistance) of the portion 112corresponding to magnetic field strength levels of the magnetizationpattern in the medium 102 associated with the data such as the bits. Asdiscussed above, ΔR/R=Δρ/ρ so resistivity and resistance are effectivelyinterchangeable with respect to sensor 114. System 100 can furtherinclude a controller 116, of which the sensor 114 can be part, whichcontrols the write head 104 and the read head 106 and which converts thedetected resistivity/resistance of the portion of GMR material 112 to asignal (for example, a digital signal) which is indicative of the storeddata on the medium 102 (for example, stored bits).

System 100 can further include a drive mechanism, depicted schematicallyby arrows and boxes 118, which causes the medium 102 to move past theread head 110 and the write head 104. It is to be understood that drivemechanism 118 can be any suitable mechanism known in the art andappropriate to the character of the medium 102; for example, a tapedrive type mechanism, a disk drive type mechanism, and the like.

It will be appreciated that resistivity/resistance sensor 114 can employany of a number of well-known techniques to sense theresistivity/resistance of portion 112 of the GMR material. For example,suitable bridge circuits can be employed in order to determine theresistance of the portion 112, and knowing the shape and dimensions ofthe portion 112, the resistivity can then be determined. As discussedabove, the percent change in both quantities is identical for a givengeometry. The construction of resistivity/resistance sensor 114 andcontroller 116 can be similar to those currently employed in prior-artsystems utilizing the AMR effect. Typically, in digital applications, acomparator circuit is employed to examine the voltage drop across theportion 112 of GMR material for a suitable clipping level. Appropriateclocking circuitry, as is known in the art, can also be included incontroller 116.

It should be noted that digital magnetic recording systems employing themagnetoresistance effect do not need to rely on motion to produce atime-changing magnetic flux as in older systems. Prior-art inductiveread heads produce a voltage in the pickup coil which is proportional tothe time rate of change of magnetic flux (dφ/dt). Since the z-componentof the H field present on magnetic recording medium 102 is senseddirectly, system 100 can work with no motion or with slow motion. Thiswould be desirable, for example, in a video cassette recorder wherein itwould no longer be necessary to spin the head to produce a large dφ/dtin order to achieve “freeze-frame.” As noted above, read heads accordingto the present invention can be employed in many different types ofdevices, including computer disk drives, video cassette recorders,digital and analog audio tape decks, “minidisk” playback systems,magnetic card readers such as credit card readers, and the like. Anytype of digital or analog magnetic storage readout can be accomplished.Other applications unrelated to magnetic storage include servo systemssuch as automatic braking systems for autos, non-destructive testing(detection of eddy currents around defects), and the like.

Still with reference to FIG. 12, a method of detecting magnetic fieldstrength according to the present invention will now be discussed. Themagnetic field strength can be that of a magnetic field associated withany medium, for example, the magnetic recording medium 102. One step ofthe method includes providing a sensing head, such as read head 110which comprises a portion 112 of a magnetoresistive material exhibitingthe GMR. The material can be any of the materials discussed above. Themethod can also include exposing the sensing head such as read head 110to the magnetic field of the medium such as magnetic recording medium102 and sensing electrical resistivity of the portion 112 of thematerial exposed to the magnetic field. The resistivity varies due tothe magnetic field.

It is to be understood that throughout this discussion and the foregoingdiscussion of the magnetic recording system 100, no explicit value forthe resistivity of portion 112 need necessarily be calculated; forexample, as discussed above, a voltage drop which depends on theresistivity can be used as a clipping level or threshold withoutexplicit calculation of the resistivity. The method can also includeconverting the sensed electrical resistivity (or bulk resistance) of theportion 112 to a signal which is indicative of the magnetic strength ofthe magnetic field (or magnetization pattern) associated with the mediumsuch as magnetic recording medium 102. For example, an analog signalcorresponding to the voltage drop across the portion 112 can beprocessed using the aforementioned comparator circuitry to produce adigital signal which corresponds to the sequence of bits (i.e., 1's and0's) on magnetic recording medium 102. Instead of a magnetic recordingmedium 102, the medium could be a material in an auto braking system ora material undergoing non-destructive testing(NDT). The magnetizationpattern could be a series of bits but can be, for example, an analogpattern, a pattern for an auto brake system, or a pattern associatedwith the eddy currents around defects in NDT.

Still with reference FIG. 12, it is to be appreciated that portion 112can be coupled to resistivity sensor 114 using suitable leads 120, 122.The ends of leads 120, 122 can be located on portion 112 such that thereis a defined geometry between the ends of the leads, permitting easydetermination of the resistance (and corresponding voltage drop) due tochanges in the resistivity. Windings 106 of write head 104 can also becoupled to controller 116 as shown.

EXAMPLE

Amorphous Tb_(x)Co_(1−x) thin films with the compositions of 25˜32atomic % of Tb were prepared using facing target magnetron sputteringwith a composite target. The parameter “x” is the atomic fraction of Tb,ranging from 0.25 to 0.32. The film compositions were controlled bychanging the Ar sputtering gas pressure (5˜15 mTorr). In order to inducethe phase separation of Co and TbN, nitrogen was introduced intoamorphous TbCo thin films by annealing at 650° C. for 12 hours with acontinuous flow of 10% H₂-balance N₂ gas mixture. (Annealing at a lowertemperature, such as 400° C., would be possible with a longer dwelltime). The pressure of the gas mixture in the annealing furnace wasabout 1 atmosphere and the gas was flowed at a rate of approximately 500standard cc/minute. The pressure and flow rate of the gas are notcritical control parameters. Phase analysis was made with x-raydiffraction (See FIG. 11) and with secondary electron images on a fieldemission SEM. Magnetization loops were made at room temperature infields up to 13 kOe using a vibrating sample magnetometer (VSM). Themagnetization loops and magnetization vs. temperature with an appliedfield up to 30 kOe were measured at temperatures from 20 K to 300 Kusing a SQUID magnetometer. Magnetoresistance at room temperature wasmeasured with applied fields up to 8.5 kOe with a DC electromagnet usingVan der Pauw geometry. Electrical contacts were made with fine wiresattached using silver paste at the corners of a square sample. Themagnetoresistance versus temperature was measured from 20 K to 250 K at40 kOe in a superconducting coil cryostat. The Hall effect is largebefore annealing when the alloy is in the amorphous state.

Secondary electron SEM imaging shows that TbN becomes the primary phaseand precipitates out of a Co matrix. The TbN forms large (>20 μm)surface patches and small(<0.5 μm) precipitates in the Co matrix. Thesmall precipitates probably segregate to the grain boundaries of the Comatrix. The X-ray diffraction pattern from the film annealed at 650° C.exhibits strong peaks of TbN and the main peaks of the fcc and hcp Costructures which indicates that the Co matrix includes two structures,as seen in FIG. 11. Both the Co and TbN phases are crystalline after theheat treatment, though the crystal structure of the Co is somewhatimperfect, possibly due to beginning with an amorphous material.

Various features of the magnetic and magnetotransport propertiesobtained for the Co—TbN with 32% TbN composition are shown in FIGS. 1,13, 14, 15A and 15B. The MR data are typical of the behavior for allcompositions from 25 mole % to 32 mole % TbN. The increase inmagnetization with temperature and the broad minimum in magnetization atan applied field of 1 kOe are indications of ferrimagnetic behavior(FIG. 13). The small jump in the magnetization curves at fields of 10,20 and 30 kOe at 50 K indicate that at these high fields the magneticmoments of the TbN precipitates are ferromagnetically aligned with theCo matrix. The Curie temperature of TbN can be estimated as about 75 Kby extrapolating from the break in the magnetization curve at 30 kOefield. FIG. 14 shows the magnetization curves of Co_(0.68)—(TbN)_(0.32)film with an applied field at various temperatures. The magnetization isnot fully saturated even at the highest applied field of 30 kOe. FIGS.15A and 15B show the resistivity (ρ) and resistivity change (δρ) ofCo_(0.68)—(TbN)_(0.32) as a function of magnetic field at roomtemperature. The curve shows a cusp-type negative magnetoresistance atroom temperature: the decrease of resistivity with increasing appliedfield. Considering that the electrical properties of the rare-earthnitrides DyN, HoN, and ErN are all metallic, N. Sclar, J. Appl. Phys.1534 (1964), and that the TbN has the same electronic structure as thosenitrides, it can be inferred that the TbN precipitate is also anordinary resistivity metal. The magnetoresistivity (δρ) andmagnetoresistance (δρ/ρ) of Co_(0.68)—(TbN)_(0.32) are about 1.12×10⁻⁷Ωcm and 0.72% at room temperature up to 8 kOe field, respectively, wherethe sign of the magnetoresistance is negative (FIG. 1).

The GMR effect of Co—TbN macroscopic ferrimagnets can be described interms of the scattering of spin polarized conduction electrons by theantiparallel exchange coupled spins at the phase boundary between theTbN precipitates and the Co matrix. R. J. Gambino and J. Wang, supra. Inthe Co—TbN system the matrix and precipitate are both metallic. Thematrix is ferromagnetic and the precipitates are magnetically orderedthrough the exchange with the Co matrix. In the Co—EuS system, the Comatrix is metallic but the EuS in the particles is a semiconductor.Therefore, the carriers are mainly confined to the Co matrix, which isthe main difference between Co—EuS and Co—TbN systems. In the Co—EuSsystem the conduction electrons are scattered mainly at the Co—EuSinterface whereas in the Co—TbN system scattering can occur both at theCo/TbN interface and in the TbN precipitates. That scattering depends onthe magnetic alignment of EuS with respect to the cobalt. In the Co—TbNsystem, when the carriers pass through the phase boundary between thetwo metallic phases, Co and TbN, they are scattered by the antiparallelexchange coupled spin and the resistivity is high in zero or low fields.In high fields with the Co and TbN ferromagnetically aligned, this spinscattering contribution is expected to disappear.

The resistance change of Co_(0.68)—(TbN)_(0.32) with temperature at H=0and 40 kOe displays a behavior typically observed in metals, as shown inFIG. 2. The GMR of Co_(0.68)—(TbN)_(0.32) in the high field of 40 kOe isaround 9% at 250 K (FIG. 16), which is due to the increase of theferromagnetic alignment between the Co and TbN by the high field. TheGMR of Co—TbN shows an increase with temperature (FIG. 16). These datawere obtained from the temperature dependence of resistance shown inFIG. 2. In contrast, ordinary GMR materials have a negativemagnetoresistance, where the magnetoresistance decreases with increasingtemperature, as described in R.J. Gambino et al., 75 J. Appl. Phys. 6909(1994). Based on the magnetization curve with temperature at an appliedfield of 1 kOe in FIG. 13 , the magnetization decreases with decreasingtemperature, which can be explained by the increase of magnetization ofthe antiparallel exchange coupled TbN phase. The antiparallel exchangecoupling between the Co and TbN with different magnetic moments may alsobecome stronger with decreasing temperature. As a result, theferrimagnetic behavior between the two different magnetic momentsincreases with decreasing temperature and thus the magnetizationdecreases.

The magnetization behavior with applied field can be divided by twodifferent regions around 150 K Even though the magnetization curvesshown in FIG. 14 are extrapolated to higher fields, the magnetizationmay not be saturated in a 40 kOe field at temperatures less than 150 KOn the other hand, at high temperatures the magnetization approachessaturation in a 40 kOe field. When the temperature increases above 150K, the antiparallel exchange coupling of the Co and TbN decreases withincreasing temperature and the Co and TbN are more easily alignedferromagnetically in a field. Therefore, the spin scatteringcontribution is expected to decrease with high applied fields withincreasing temperature. Thus the magnetoresistivity, δρ, and the GMR,δρ/ρ, both increase with increasing temperature.

FIG. 3 shows the dependence of GMR on the volume % of TbN calculatedfrom the composition assuming the normal densities for TbN and Co. Theroom temperature GMR (δρ/ρ) of a Co_(1−x)—(TbN)_(x) system at 8 kOefield increases with the TbN volume fraction. The 32 atomic % TbNcomposition which corresponds to 39 volume % of TbN has the largest roomtemperature GMR.

Thus, the new macroscopic ferrimagnet, Co—TbN, including TbNprecipitates in a cobalt matrix. has been formed by the transformationof amorphous TbCo to crystalline Co and TbN phases induced by annealingin an N₂ gas atmosphere. The fully transformed films annealed at 650° C.demonstrate typical macroscopic ferrimagnetic properties: evidence ofnegative exchange, magnetic compensation and negative giantmagnetoresistance. The antiparallel exchange coupling at the phaseboundary between the TbN precipitates and the Co matrix can explain allof these observations. It was found that the TbN magnetization and/orthe ferrimagnetic exchange coupling at the phase boundary was increasedwith decreasing temperature. The temperature dependence of resistivityof the macroscopic ferrimagnet Co_(0.68)—(TbN)_(0.32) shows the typicaltemperature dependence of a metal. The Co_(0.68)—(TbN)_(0.32) system hasthe largest values of δρ and δρ/ρ in the composition range of 25 to 32%TbN. The GMR effect was observed to increase with increasing temperaturein the range 30 to 230 K, which was believed to be ascribed to theincrease of ferromagnetic alignment of the Co and TbN with a fieldcaused by the weakening of exchange coupling by temperature.

Although the present invention has been described with reference tospecific exemplary embodiments, it should be understood that variouschanges, substitutions and alterations can be made to the disclosedembodiments without departing from the spirit and scope of the inventionas defined by the appended claims.

What is claimed is:
 1. A magnetoresistive material exhibiting the giantmagnetoresistance effect (GMR) and having two phases, said materialcomprising: (a) a first phase comprising a matrix of an electricallyconductive ferromagnetic transition metal or an alloy thereof; and (b) asecond precipitate phase comprising an electrically conductive rareearth pnictide, wherein said electrically conductive rare earth pnictideexhibits ferromagnetic behavior in a precipitated form when precipitatedout of said matrix, said second phase being antiferromagneticallyexchange coupled to said first phase.
 2. The material of claim 1,wherein: said electrically conductive ferromagnetic transition metalcomprises at least one of iron, cobalt and nickel.
 3. The material ofclaim 2, wherein: said rare earth pnictide comprises a rare earthelement compounded with at least one of nitrogen, phosphorous, arsenic,antimony and bismuth.
 4. The material of claim 1, wherein saidprecipitate phase exhibits independent ferromagnetic behavior.
 5. Thematerial of claim 1, wherein said precipitate phase does not exhibitindependent ferromagnetic behavior but only exhibits ferromagneticbehavior when precipitated out of said matrix.
 6. The material of claim1, wherein: said matrix comprises cobalt; and said precipitate phasecomprise terbium nitride (TbN) in a volume percent sufficient to producethe GMR in said material without causing discontinuity in said cobaltmatrix.
 7. The material of claim 6, wherein: said cobalt matrix has avolume percent of from about 30% to about 75%; and said terbium nitrideprecipitate phase has a volume percent of from about 25% to about 70%.8. The material of claim 6, wherein: said cobalt matrix has a volumepercent of from about 61% to about 70%; and said terbium nitrideprecipitate phase has a volume percent of from about 30% to about 39%.9. A magnetoresistive material exhibiting the giant magnetoresistanceeffect (GMR) and having two phases, said material comprising: (a) afirst phase comprising a matrix of an electrically conductiveferromagnetic transition metal or an alloy thereof; and (b) a secondprecipitate phase comprising an electrically conductive Heusler alloy,wherein said electrically conductive Heusler alloy exhibitsferromagnetic behavior in a precipitated form when precipitated out ofsaid matrix, said second phase being antiferromagnetically exchangecoupled to said first phase.
 10. The material of claim 9, wherein: saidmatrix comprises cobalt; and said precipitate phase comprises Co₂MnSn ina molar percent sufficient to produce the GMR in said material withoutcausing discontinuity in said cobalt matrix.
 11. The material of claim10, wherein said molar percent of Co₂MnSn ranges from about 16.7 mole %to about 50 mole %.
 12. The material of claim 9, wherein: said matrixcomprises cobalt; and said precipitate phase comprises Co₂TiSn in amolar percent sufficient to produce the GMR in said material withoutcausing discontinuity in said cobalt matrix.
 13. The material of claim12, wherein said molar percent of Co₂TiSn ranges from about 16.7 mole %to about 50 mole %.
 14. A method of manufacturing a magnetoresistivematerial exhibiting the giant magnetoresistance effect (GMR) and havingtwo phases, said method comprising the steps of: (a) providing a targetof an electrically conductive ferromagnetic transition metal or an alloythereof; (b) locating a plurality of pellets of an electricallyconductive rare earth element on a surface of said target; (c)sputtering said target and said pellets to cause the deposition on asubstrate of an amorphous alloy of said electrically conductiveferromagnetic transition metal or alloy thereof, and said electricallyconductive rare earth element; and (d) annealing said amorphous alloy inan atmosphere containing an element from column VA of the CAS periodictable, or a compound thereof, to cause said element or compound thereofto bond to said rare earth element and form a precipitate phase of arare earth pnictide within a matrix of said ferromagnetic transitionmetal or said alloy thereof.
 15. The method of claim 14, wherein: step(a) comprises providing said target as a target of cobalt; step (b)comprises providing pellets of terbium; and step (d) comprises annealingin an atmosphere containing nitrogen to form said precipitate phase as arare earth nitride.
 16. The method of claim 15, wherein, in step (c),said sputtering is performed to yield said amorphous alloy with acomposition of from about 10 atomic percent to about 50 atomic percentterbium with a balance comprising said cobalt.
 17. The method of claim16, wherein in step (c), said sputtering comprises facing targetsmagnetron sputtering with an argon plasma.
 18. A method of detectingmagnetic field strength of a magnetization pattern in a medium, saidmethod comprising the steps of: (a) providing a sensing head comprisinga portion of a magnetoresistive material exhibiting the giantmagnetoresistance effect and having two phases, said material in turncomprising: (a-1) first phase comprising a matrix of an electricallyconductive ferromagnetic transition metal or an alloy thereof; and (a-2)a second precipitate phase comprising an electrically conductive rareearth pnictide, wherein said electrically conductive rare earth pnictideexhibits ferromagnetic behavior in a precipitated form when precipitatedout of said matrix, said second phase being antiferromagneticallyexchange coupled to said first phase; (b) exposing said sensing head tothe magnetic field of the magnetization pattern in the medium; (c)sensing electrical resistivity of said portion of said material exposedto the magnetic field of the magnetization pattern in the medium; and(d) converting said sensed electrical resistivity of said portion to asignal indicative of the magnetization pattern in the medium.
 19. Amethod of detecting magnetic field strength of a magnetization patternin a medium, said method comprising the steps of: (a) providing asensing head comprising a portion of a magnetoresistive materialexhibiting the giant magnetoresistive effect and having two phases, saidmaterial in turn comprising: (a-1) a first phase comprising a matrix ofan electrically conductive ferromagnetic transition metal or an alloythereof; and (a-2) a second precipitate phase comprising an electricallyconductive Heusler alloy, wherein said electrically conductive Heusleralloy exhibits ferromagnetic behavior in a precipitated form whenprecipitated out of said matrix, said second phase beingantiferromagnetically exchange coupled to said first phase; (b) exposingsaid sensing head to the magnetic field of the magnetization pattern inthe medium; (c) sensing electrical resistivity of said portion of saidmaterial exposed to the magnetic field of the magnetization pattern inthe medium; and (d) converting said sensed electrical resistivity ofsaid portion to a signal indicative of the magnetization pattern in themedium.
 20. A magnetic recording system adapted for use with a magneticrecording medium having a coercive force and adapted to store datatherein, the data being stored in the form of a magnetization patternrecorded in the medium, said system comprising: (a) a read head forreading the recorded magnetization pattern in the medium, said read headcomprising: (a-1) a portion of a magnetoresistive material exhibitingthe giant magnetoresistance effect (GMR) and having two phases, saidmaterial in turn comprising: (a-1a) a first phase comprising a matrix ofan electrically conductive ferromagnetic transition metal or an alloythereof; and (a-1b) a second precipitate phase comprising anelectrically conductive rare earth pnictide, wherein said electricallyconductive rare earth pnictide exhibits ferromagnetic behavior in aprecipitated form when precipitated out of said matrix, said secondphase being antiferromagnetically exchange coupled to said first phase,said magnetoresistive material being located in proximity to the medium;and (a-2) a resistivity sensor which detects resistivity of said portioncorresponding to magnetic field strength levels adjacent the mediumassociated with the recorded magnetization pattern; and (b) a controllerwhich controls said read head and which converts said detectedresistivity of said portion to a signal indicative of the stored data inthe medium.
 21. The system of claim 20, further comprising a drivemechanism which causes the medium to move past said read head.
 22. Thesystem of claim 21, further comprising a write head for writing the datato the medium by producing a magnetic field of N→S or S→N polarityexceeding the coercive force of the magnetic medium so as to record themagnetization pattern therein, wherein said drive medium also causes themedium to move past said write head.
 23. The system of claim 20, whereinsaid data is digital data comprising a plurality of bits and saidcontroller provides a digital signal indicative of the stored bits inthe medium.
 24. A magnetic recording system adapted for use with amagnetic recording medium having a coercive force and adapted to storedata therein, the data being stored in the form of a magnetizationpattern in the medium, said system comprising: (a) a read head forsensing the magnetization pattern recorded in the medium, said read headcomprising: (a-1) a portion of a magnetoresistive material exhibitingthe giant magnetoresistance effect (GMR) and having two phases, saidmaterial comprising: (a-1a) a first phase comprising a matrix of anelectrically conductive ferromagnetic transition metal or an alloythereof; and (a-1b) a second precipitate phase comprising anelectrically conductive Heusler alloy, wherein said electricallyconductive Heusler alloy exhibits ferromagnetic behavior in aprecipitated form when precipitated out of said matrix, said secondphase being antiferromagnetically exchange coupled to said first phase,said magnetoresistive material being located in proximity to the medium;and (a-2) a resistivity sensor which detects resistivity of said portioncorresponding to magnetic field strength levels adjacent the mediumassociated with the recorded magnetization pattern; and (b) a controllerwhich controls said write head and said read head and which convertssaid detected resistivity of said portion to a signal indicative of thestored data in the medium.
 25. The system of claim 24, furthercomprising a drive mechanism which causes the medium to move past saidread head.
 26. The system of claim 25, further comprising a write headfor writing the data to the medium by producing a magnetic field of N→Sor S→N polarity exceeding the coercive force of the magnetic medium soas to record the magnetization pattern therein, wherein said drivemedium also causes the medium to move past said write head.
 27. Thesystem of claim 24, wherein said data is digital data comprising aplurality of bits and said controller provides a digital signalindicative of the stored bits in the medium.
 28. A method ofmanufacturing a magnetoresistive material exhibiting the giantmagnetoresistance effect (GMR) and having two phases, said methodcomprising the steps of: (a) providing a target of an electricallyconductive ferromagnetic transition metal or an alloy thereof; (b)locating a plurality of pellets of Heusler alloy components on a surfaceof said target; (c) sputtering said target and said pellets to cause thedeposition on a substrate of an amorphous alloy of said electricallyconductive ferromagnetic transition metal or alloy thereof, and saidHeusler alloy components; and (d) annealing said amorphous alloy tocause formation of a precipitate phase of a Heusler alloy within amatrix of said ferromagnetic transition metal or said alloy thereof. 29.The method of claim 28, wherein: step (a) comprises providing saidtarget as a target of cobalt; and step (b) comprises providing pelletsof Mn and Sn.
 30. The method of claim 29, wherein in step (c), saidsputtering comprises facing targets magnetron sputtering with an argonplasma.
 31. The method of claim 28, wherein: step (a) comprisesproviding said target as a target of cobalt; and step (b) comprisesproving pellets of Ti and Sn.
 32. The method of claim 31, wherein instep (c), said sputtering comprises facing targets magnetron sputteringwith an argon plasma.